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Mechanics and dynamics of circular milling operations Kardes, Nimet

Abstract

This thesis presents modeling of the mechanics and dynamics of circular milling operations. With the recent advances in CNC machine tools which have high contouring accuracy, the circular milling operations are used in high speed opening of pockets in die, mold and aerospace machining industry. While the cutter rotates around the spindle axis, it follows a circular-trochoidal path, avoiding momentary pauses to change feed directions. The cutter engagement conditions, hence the chip thickness, the cutting force directions and amplitudes, and the dynamic stability of the milling process continuously change in circular milling operations. This thesis presents the first research in modeling the mechanics and dynamics of circular milling operations in the literature. The kinematics of the chip removal generation is first modeled by considering the rigid body motions of the cutter and cutting edges. The time varying chip load and the resulting milling forces are predicted with experimental validation. The dynamic stability of the process is complicated by three factors. The system dynamics has two delay terms and two periodic behaviours. Additionally the parameters of the coupled differential equations have time varying coefficients. First, the stability of the system is solved by taking the averages of the periodic coefficients in the frequency domain. The stability law developed by Altintas and Budak are extended to the circular milling. Two alternative methods were studied to improve the frequency domain stability solution. The direct method proposed by Olgac and Sipahi, converged to the frequency domain solution since the assumptions were identical. The Time Finite Element method proposed by Stepan, Bayly and Mann is a numerical, time domain method where the time varying directional coefficients can be considered. To simplify the time finite element solution and decrease the computation time, only the most flexible mode in each direction was taken into account. The experiments were conducted to verify the proposed dynamic models and the simulation results obtained from frequency domain solution and time finite element method were compared against experimental results. Both methods gave reasonable results only for speed independent and low axial depth of cut region but they are not able to predict the stability of a circular milling operation accurately.

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